Wellbores are formed in subterranean formations for various purposes including, for example, the extraction of oil and gas from a subterranean formation and the extraction of geothermal heat from a subterranean formation. A wellbore may be formed in a subterranean formation using a drill bit, such as, an earth-boring rotary drill bit. Different types of earth-boring rotary drill bits are known in the art, including, for example, fixed-cutter bits (which are often referred to in the art as “drag” bits), rolling-cutter bits (which are often referred to in the art as “rock” bits), impregnated bits (impregnated with diamonds or other superabrasive superabrasive particles), and hybrid bits (which may include, for example, both fixed cutters and rolling cutters).
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. The drill string may comprise a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface of the formation. When weight is applied to the drill string and consequently to the drill bit, the rotating bit engages the formation and proceeds to form a wellbore. The weight used to push the drill bit into and against the formation is often referred to as the “weight-on-bit” (WOB). As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the wellbore. A diameter of the wellbore farmed by the drill bit may be defined by the cutting structures disposed at the largest outer diameter of the drill bit.
Different types of bits work more efficiently against formations having different hardnesses. For example, bits containing inserts that are designed to shear the formation, such as fixed-cutter bits, frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDCs) as their cutting faces.
Roller cone bits are efficient and effective for drilling through formation materials that are of medium to high hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in which the inserts of the rotating cones are impacted against the formation material. This action compresses the material beyond its compressive strength and allows the bit to cut through the formation.
For still harder formation materials, the mechanism commonly used for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred, such as diamond-impregnated bits. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations. For these types of formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are generally more effective.
During abrasive drilling with a diamond-impregnated bit, diamonds or other superabrasive particles scour or abrade away concentric grooves while the rock formation adjacent the grooves is fractured and removed. Conventional impregnated drill bits typically employ a cutting face composed of superabrasive cutting particles, such as natural or synthetic diamond grit, randomly dispersed within a matrix of wear-resistant material. These diamond particles may be cast integrally with the body of the bit, as by low-pressure infiltration, or may be preformed separately, as by a hot isostatic pressure (HIP) process, to form so-called “segments” which are attached to the bit by brazing or furnaced to the bit body during manufacturing thereof by an infiltration process.
Diamond-impregnated bits may be formed by any one of a number of powder metallurgy processes known in the art. During the powder metallurgy process, abrasive particles (e.g., diamond) and a matrix powder (e.g., tungsten carbide (WC) powder) are placed in a desired location in a mold cavity proximate a wall thereof and infiltrated with a molten binder material (e.g., a copper alloy). Upon cooling, the bit body includes the binder material, matrix material, and the abrasive particles suspended both near and on the surface of the drill bit. The abrasive particles typically include small particles of natural or synthetic diamond. Synthetic diamond used in diamond impregnated drill bits is typically in the form of single crystals. However, thermally stable polycrystalline diamond (TSP) elements may also be used.
With respect to the diamond-impregnated material to be incorporated in the bit, diamond granules are formed by mixing diamonds with matrix powder and binder into a paste. The paste is then packed into the desired areas of a mold. The resultant diamond-impregnated portions of the bit often have irregular diamond distribution, with areas having a cluster of too many diamonds and other areas having a lower diamond concentration, or even a void—an area free of diamonds. The diamond clusters may lack sufficient matrix material around them for good diamond retention. The areas devoid of, or low in, diamond concentration may have poor wear properties. Accordingly, bits with uncontrolled diamond distributions may fail prematurely due to uneven wear or fracturing.
Previous attempts to solve the problem of uncontrolled diamond distribution include encapsulating individual diamond granules in a metal matrix material to form particles, each with a diamond granule in the center and an outer shell of metal. Then the encapsulated diamonds are mixed with a powder metal matrix and binder to form the paste, as described above. One example of a similar approach is found in U.S. Pat. No. 7,350,599 to Lockwood et al., issued Apr. 1, 2008. In this way, the individual diamond granules are less likely touch each other or cluster together and are more evenly distributed throughout the resulting paste and diamond-impregnated portions of the drill bit.